Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

libcef/browser/content_browser_client.h

@@ -53,6 +53,8 @@ class CefContentBrowserClient : public content::ContentBrowserClient {
   std::unique_ptr<base::Value> GetServiceManifestOverlay(
       base::StringPiece name) override;
   std::vector<ServiceManifestInfo> GetExtraServiceManifests() override;
+  bool IsSameBrowserContext(content::BrowserContext* context1,
+                            content::BrowserContext* context2) override;
   void AppendExtraCommandLineSwitches(base::CommandLine* command_line,
                                       int child_process_id) override;
   content::QuotaPermissionContext* CreateQuotaPermissionContext() override;